1. Field of the Invention
The present invention relates to a photothermographic material preferably used in the field of films for medical diagnosis, the field of films for graphic arts, or the like.
2. Description of the Related Art
In recent years, in the medical field and the graphic arts field, there has been a strong desire for providing a dry photographic process from the viewpoints of protecting the environment and economy of space. Further, the development of digitization in these fields has resulted in the rapid development of systems in which image information is captured and stored in a computer, and then when necessary processed and output by transmitting it to a desired location. Here the image information is output onto a photosensitive material using a laser image setter or a laser imager, and developed to form an image at the location. It is necessary for the photosensitive material to be able to record an image with high-intensity laser exposure and that a clear black-tone image with a high resolution and sharpness can be formed. While various kinds of hard copy systems using pigments or dyes, such as ink-jet printers or electrophotographic systems, have been distributed as general image forming systems using such digital imaging recording materials, images on the digital imaging recording materials obtained by such general image forming systems are insufficient in terms of the image quality (sharpness, granularity, gradation, and tone) needed for medical images used in making diagnoses, and high recording speeds (sensitivity). These kinds of digital imaging recording materials have not reached a level at which they can replace medical silver halide film processed with conventional wet development.
Photothermographic materials utilizing organic silver salts are already known. Photothermographic materials have an image forming layer in which a reducible silver salt (for example, an organic silver salt), a photosensitive silver halide, and if necessary, a toner for controlling the color tone of developed silver images are dispersed in a binder.
Photothermographic materials form black silver images by being heated to a high temperature (for example, 80° C. or higher) after imagewise exposure to cause an oxidation-reduction reaction between a silver halide or a reducible silver salt (functioning as an oxidizing agent) and a reducing agent. The oxidation-reduction reaction is accelerated by the catalytic action of a latent image on the silver halide generated by exposure. As a result, a black silver image is formed in the exposed region. Photothermographic materials have been described in many documents, and the Fuji Medical Dry Imager FM-DPL is an example of a practical medical image forming system using a photothermographic material that has been marketed.
These photothermographic materials utilizing an organic silver salt have a great characteristic of containing all components necessary for image formation in the film in advance and being capable of forming images only by heating. However, on the other hand, there are many technical problems to be solved.
Generally, silver halide photographic materials contain dyes for various purposes. However, the dyes are mainly employed for two purposes described below.
The first purpose is to provide a light filter, an antihalation effect or an anti-irradiation effect to the materials, where the dye exerts its function during the imagewise exposure process but becomes unnecessary after an image is formed. In the case of a dye having absorption in the visible light region, when the dye remains, the remaining dye degrades image quality. Therefore, it is necessary to decolor the remaining dye during the developing process.
The second purpose is to control image tone, where the dye exerts its function after the image forming process, and it is required to complement the color tone of developed silver images and to have a desired spectral light absorption characteristic in order to bring about preferable image tone.
A dye which is used for a photothermographic material has another task, in addition to the above purposes, which is unique to the photothermographic material. As mentioned above, the photothermographic material contains all components necessary for forming an image in the film in advance. Further, even after an image has been formed, unreacted components or reaction products remain in the film, and these exert complicated influences on storage stability of the dye (prior to or after the image formation). Especially after the image formation, it is required that the dye is stable so that change in color or decoloration does not occur during image storage under various conditions (for example, under light illumination, high temperature and high humidity conditions, or the like).
More specifically, in order to attain images with a good degree of sharpness, the incorporation of dyes is very important for photothermographic materials exposed by a laser beam to provide sufficient antihalation and anti-irradiation effects at the wavelength used for the imagewise exposure. As for the wavelength of a laser beam used for the exposure, a wide range of wavelength regions such as the near infrared region, the infrared region, or the visible light region from red to blue can be applied.
For photothermographic materials exposed with either a near infrared or an infrared laser beam, Japanese Patent Application Laid-Open (JP-A) Nos. 9-146220 and 11-228698 disclose photothermographic materials which practically require no color bleaching mechanism therein due to use of a dye which has an absorption maximum within the near infrared region outside of visual sensitivity, a narrow half band width, and little light absorption within the visible light region. All of the patents, patent publications, and non-patent literature cited in the specification are hereby expressly incorporated by reference herein.
For photothermographic materials which are subjected to imagewise exposure with a laser beam having a wavelength within the visible light region of blue to red, a method for decoloring dyes by way of heating during a thermal developing process has been proposed. For example, U.S. Pat. No. 5,135,842 discloses a method for decoloring polymethine dyes of a specific structure by heating. Moreover, U.S. Pat. Nos. 5,314,795, 5,324,627, and 5,384,237 disclose methods in which polymethine dyes are decolorized by heating using a carbanion generating agent.
However, the discoloring mechanisms described above often bring about problems such as incomplete decoloring of dyes or dye decolorization during storage of photothermographic materials due to the insufficient stability of dye occurring after bleaching ability has been enhanced. Moreover, there are also problems such as film turbidity caused by crystallization of decoloring reaction products in the film and powdery deposits oozing out to the surface. Especially, in photothermographic materials used in medical diagnosis, high sharpness and preferable image tone are required. Furthermore, demand has increased for image forming methods used for processing photothermographic materials at a higher speed within a short time.
As dyes used for photographic applications, metal phthalocyanine dyes are well known in the art, and in particular, water-soluble metal phthalocyanine dyes and pigments are known. JP-A No. 2003-295388 discloses the use of a water-soluble metal phthalocyanine dye in photothermographic materials. However, these conventional metal phthalocyanine dyes have a broad spectral light absorption spectrum and can only be added in an amount in a range that does not affect image tone when used in a photothermographic material, and thus, sufficient antihalation effect has not been achieved.
JP-A Nos. 10-268465, and 2000-39685 disclose photothermographic materials which contain dyes or pigments having an absorption maximum in the wavelength region of from 500 nm to 700 nm, or from 520 nm to 580 nm. However, these dyes present problems such as having a broad light absorption spectrum or insufficient storage stability.